Sulfur dioxide at the Venus cloud tops, 1978–1986

Ultraviolet spectroscopy from the Pioneer Venus Orbiter shows a decline in the cloud top abundance of SO 2 from about 100 ppb to about 10 ppb in the period 1978–1986. A consistent decline in polar haze has occurred over the same period, with the correlation coefficient between these two observables of r = 0.8. Star calibrations determine the instrument sensitivity to within 10%, which rules out the possibility that this is an instrumental effect. Systematic errors could increase the SO 2 abundance to twice the inferred values in later orbits. Tracking of SO 2 features and power spectral analysis give rotation periods for the longer‐lived features of 3.6–5.2 days, consistent with cloud‐tracked winds observed at other wavelengths. The behavior of SO 2 and polar haze can be plausibly explained by episodic injection of SO 2 into the cloud top regions, for example, by active volcanism.